Rapid changes in air pressure that occured during Thursday morning's severe weather may have resulted in a meteotsunami at Navy Pier with the water fluctuating two feet during the event.
The National Weather Service is currently trying to determine if a meteotsunami took place on Lake Michigan at Chicago's Navy Pier early this morning. This was following examination of a water guage at the site which indicated that there had been a major fluctuation in water levels in the area as severe weather hit.
According to a storm report, the gauge which was operated by the U.S. Geological Survey showed that water levels in the lake went up somewhere between 18 inches and two feet within one-hour around the Chicago Lock. A similar fluctuation was observed at Calumet Harbor around the same time, said officials.
Water level fluctuations have continued to occur on Lake Michigan Thursday, and into Friday and red flags are still flying at beaches as dangerous swim conditions are expected to exist through at least Friday and possibly into. Walking along the beaches in East Rogers Park this, you'd never know there had been any bad weather. Despite the red flags and lifeguards constantly trying to keep people out of the water, there are families with small children trying to get in for a quick swim before the parents need to leave for work.
I overheard one family arguing with the lifeguard, claiming that they'd heard reports of a tsunami on the lake and if that was why they weren't allowed in it was ridiculous, saying that there was no such thing as a tsunami on a lake. The lifeguard patiently explained that these kinds of tsunamis weren't like the ones you see in movies or on t.v. Instead of a great big wave that wipes out everything, they are typically smaller, don't come from earthquakes but changes in air pressure and cause water levels to rise over minutes to an hour. But what the lifeguard wanted the family to understand was that it wasn't the waves that were necessarily the biggest danger. It was the rip tides that resulted after the storm had passed. The family didn't seem convinced, but packed up anyway and left with a crying child who pulled at his parent's hand, wanting to get back in the water.
The reason it was so difficult for families in the East Rogers Park to understand the impact of the potential meteotsunami was that the storms that had been predicted for the area hadn't hit the northern Chicago suburbs and both Thursday and Friday morning had been mostly sunny and hot, without any summer thunderstorms like we often get this time of year. The air hadn't suddenly changed quality, such as suddenly dropping in temperature, another occurence that isn't rare on the lakes during the summer when a storm front in in the area. But the danger that Lake Michigan is most known for is not the waves but what you can't see below the surface. These are powerful rip tides and currents which are bad normally and made worse when a meteotsunamin occurs.
What Are Meteotsunamis?
Meteotsunamis are large waves that are just beginning to be better understood. Whereas tsunamis are caused by earthquakes leading to a massive displacement of water, meteotsunamis are triggered by air-pressure disturbances that often accompany quick-moving weather events, like severe thunderstorms, squalls, and other storm fronts. The storm creates a wave that moves towards the shore, and is heightened and intensified as it reaches shallow water, which causes it to slow down.
Big meteotsunamis are relatively rare phenomenon which only occur once every three years or so on Lake Michigan, but when they hit they do so without warning usually on otherwise clear days. In these cases, the wave itself can cause injury and drownings. For example, on July 4, 1929 a 20-foot meteotsunami wave pit a pier in Grand Haven and pulled ten people out into Lake Michigan where they drowned. On June, 26, 1954, a 10 foot meteotsunami wave swept seven fisherman off a pier in Chicago. All were killed.
Most meteotsunamis are too small to notice while they are happening. It is after the storm that causes them passes by that the worst risk occurs and the most people are lost. The biggest threat from meteotsunamis are rip currents.
A 2019 study found meteotsunamis lead to rip currents that cause more severe risks and water safety hazards than were previously understood. The study showed that even when meteotsunamis form waves that are less than a foot high, they still produce strong rip currents that can pull people far out into the lake hours after the wave has passed or the increased water level has fallen. This is the reason why the most dangerous time to go back into the water is within a few hours after a storm has passed.
More meteotsunamis occur on Lake Michigan than on any other Great Lake with an average of 29 occurring each summer in Calamet Harbor which is part of the Port of Chicago.
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